1. Introduction
The field of nanotechnology has witnessed considerable progress in
recent years, both in design and the scope of applications. As an
interdisciplinary field covering diverse research areas, nanotechnology
has undoubtedly led to industrial production of novel nanomaterials with
designer properties such as being more robust, lighter, longer-lasting,
anti-reflective, antimicrobial, electrically conductive, or becoming
more luminous. In particular, nanoparticle (NP)-based medicine has
gained traction, promising to revolutionize medical treatment with
innovative therapeutics that are more potent and less
toxic1.
Nanotechnology is revolutionary, and its hype is justified, especially
for improving the quality of human life with novel consumer products
through various materials and manufacturing methods. However, there are
concerns on nanotechnology potentially creating delayed impacts on the
environment and human health, especially where detrimental consequences
are only noticed after commercialization has long begun. Thus,
technologies being developed require cautionary measures to be upheld to
avoid future predicaments for the environment and humanity, in tracking
towards a sustainable future. This is to prevent history from repeating
itself, such as petrol with lead, electronics with polychlorinated
biphenyls, chlorofluorocarbons reducing upper atmosphere ozone
dramatically, and the construction material asbestos, all of which led
to environmental disasters.
Despite substantial research in the field and considerable progress,
strategies for manufacturing nanoscale materials, through both top-down
and bottom-up production processes, still face challenges. Top-down
approaches to reduce more extended material to nanoscale dimensions
often use a number of materials leading inevitably to waste generation,
and are inappropriate for several materials2.
Traditional assembly lines create products by building them up from the
molecular level, the bottom-up strategy, which combines chemical
synthesis and self-assembly. For current practical abilities, the main
challenges are that the bottom-up strategy can be time-consuming,
requiring extensive expertise and skill to control the size, morphology,
and properties of the nanoscale products. Of paramount importance is the
choice of synthetic method to finely control these features while
circumventing uptake of impurities2. As such, the
fundamental problem regarding the bottom-up strategy is developing the
capability to exquisitely control the synthesis of the NPs while
appropriately controlling size, morphology, and properties at nanoscale
dimensions.
From a technological viewpoint, traditional methods abound in developing
processes to control the growth and properties of materials. Such bottom
up material processing at the nanoscale dimension has been developed
using channel-based microfluidic devices, albeit with some limitations.
The main drawback is insufficient mixing resulting from laminar flows,
often requiring sample dilution or reagent homogenization. The mixing
process is usually restricted to diffusion control processing under such
flows, denying the possibility of harnessing the advantages of turbulent
mixing available in macro-scale systems3. While mixing
enhancements can be achieved by incorporating multiple system
parameters, including energy input, the velocity of flow, and the
geometry of the mixers, these methods are time-consuming, leading to
cost inefficiencies. In addition, channel-based microfluidic devices can
suffer from clogging, specifically in the processing of macromolecules
or at high reactant concentrations. Furthermore, incorporating external
fields, such as electrical, magnetic, and laser fields, to control the
processing is inherently complex for channel-based microfluidic
platforms4. Although other mechanical energy forms,
including sonication and grinding or milling, are effective in materials
processing, they suffer from indiscriminate events in time and place,
such as in cavitation and uneven energy transfer, resulting in
non-uniform products. This can generate waste that, coupled with high
energy usage, limits the sustainability metrics of such processing. A
paradigm shift in microfluidics design is required to overcome such
limits.
Thin film processing is an emerging technology where the liquid is
subjected to centrifugal forces/shear stress or mechanical energy within
dynamic thin films on a surface. These forces are useful in a range of
thin film vortexing technologies, including for chemical synthesis and
separations5, material synthesis6,
material processing, lab-on-a-disc microfluidics7 and
enabling chiral selection8. Thin film processing
offers several advantages, including accelerated reaction kinetics and
improved control over chemical reactivity. The application of shear
stress presents opportunities for enabling new types of chemical
reactions and generating materials with new shapes, morphologies and
sizes.9 Thin film processing holds potential in
situations where traditional batch processing is impractical or when
conventional methods fail to provide access to unique forms of
materials.10
Rotary devices that utilize centrifugal forces, pushing away from the
rotation axis, are prime examples of how these forces can shape
interfaces and effectively control material synthesis and chemical
reactivity9. A diverse range of rotary devices have
been reviewed and shown to achieve such performances, including
lab-on-a-disc system11, spiral
seperators12, spinning disc
reactors13, and vortex fluidic
devices14. This review introduces the vortex fluidic
device (VFD) as a paradigm shift in flow processing, with scalability
factored in under the continuous-flow mode of operation of the device,
along with its utility for tuning the size, morphology, and properties
of materials at the nanoscale dimension. The VFD delivers high shear as
a constant form of mechanical energy, with tunable control over the
processing. Processing in the device is not limited by diffusion
control, providing a route to kinetically trapped novel forms of
nanomaterials, and processing can also be facilitated by applied
external fields for which the microfluidic platform is suited with the
thin film of liquid approaching uniformity of treatment as such.
Continuous-flow processing of the VFD is directly scalable, unlike batch
processing which requires precise process engineering for upscaling, in
overcoming uneven mixing and heat transfer. Whereas previous reviews of
VFDs have focused on chemical transformations15 and
comparisons with other microfluidic devices16, this
review aims to deliver additional information about the significance of
utilizing the VFD to transform material structure-property relationships
at the nanoscale with emphasis on its high green chemistry metrics.